The lymph nodes are numerous encapsulated structures occurring in mammalians along the lymphatics. They contain lymphoid cells with a precise microanatomy organized to react to a variety of antigens via humoral and cellular immune responses. Lymph nodes consist of a cortex (B-cell zone) with primary and secondary follicles, a paracortical region (T-cell zone), sinuses and medullary cords. High endothelial venules represent a specialized vasculature with specific receptors for immune cells, which facilitate the migration of lymphocytes and dendritic cells. The main function of the lymph node is to generate an efficient immune response to antigens that have penetrated the organism in the areas drained by that node. Dendritic cells present antigen to naïve B cells, with the formation of germinal centres, as well as naïve T cells in the cortex. Malignant lymphoma represents the overwhelming majority of primary lymph node neoplasms, including both Hodgkin lymphoma and non-Hodgkin lymphoma.

Key concepts

• Lymph nodes are encapsulated structures present in mammalians and distributed at specific sites throughout the body, which represent an important component of the secondary immune system.
  
• Lymph nodes have a specific microarchitecture, comprising a cortex, paracortical region, medullary cords and sinuses.
  
• The cortex is the B-cell compartment and includes primary and secondary germinal centres, whereas the paracortical zone is the T-cell compartment.
  
• Mature lymphocytes constantly recirculate between the lymph nodes and the peripheral blood, entering the lymph nodes passively via afferent lymphatics and actively via the high-endothelial venules of the lymph node via specific surface receptors.
  
• The B-cell compartment contain a range of B-cell maturation, from naïve primary follicle and mantle zone B-cells, to the centrocytes and then centroblasts of the germinal centre.
  
• The paracortical zone represents the site of T-cell activation and induction of specific T-cell migration to the follicles of the cortical region.
  
• The germinal centre represents a factory for the ultimate production of B-lineage effector cells (plasma cells and memory B cells).
  
• Most reactive diseases of the lymph node represent a form of exaggeration of the normal response.
  
• Lymphoma represents the overwhelming majority of primary lymph node neoplasms, and may be B-cell non-Hodgkin lymphoma, T-cell non-Hodgkin lymphoma or Hodgkin lymphoma, the latter representing a distinctive neoplasm of Reed–Sternberg cells, unique cells derived from B cells.